1. Technical Field
The present inventive concept relates to stabilizing outputs of voltage generators, and more particularly to a method of alternately stabilizing two generated voltages using a time-shared capacitor, an apparatus for driving a display device using a shared capacitor, and a display device having the shared capacitor.
2. Discussion of the Related Art
As mobile phone applications extend into such areas as high-resolution cameras, TV display functions, and game functions, larger volumes of information are being displayed and main screen resolution is improving to provide high-quality image displays, leading to increased use of 240×320 pixel (QVGA) resolution displays. In addition, the trend in future designs will be for the narrowest possible frame around the thinnest possible LCD panel in mobile phones, PDAs and portable media players.
A power supply in a liquid crystal display (LCD) module includes a direct-current to direct-current voltage converter (DC to DC converter), and a DC/AC backlight inverter. The DC to DC converter converts an external DC voltage from a power supply into a plurality of voltages for output to a logic circuit, and/or to a driver circuit, e.g., a gate-ON voltage VDD, a gate-OFF voltage VSS, a gamma reference voltage VREF for a data voltage, and a common voltage VCOM to an LCD panel. Generally, the voltage output to the logic circuit is about 5V or less (e.g., about 3.3V).
The internal driving circuit of the LCD outputs data to the gate lines of the LCD panel as discrete analog voltages. For example if an LCD panel displays 256 grey levels in each color pixel PX, then the internal driving circuit outputs a selected one of 256 analog voltages to each color-pixel. The 256 analog voltages are typically produced by a voltage divider comprised of a plurality of series-connected resistors.
FIG. 1B is a circuit diagram of a conventional double-throw switch SW in the internal driving circuit of a LCD connected indirectly to a pixel PX in an LCD panel 1240 of the LCD. Each pixel PX in the LCD panel 1240 of an LCD includes a liquid crystal layer that has a capacitance CLC and a storage capacitor that has capacitance Cst. A positive driving voltage Vout1/y and a negative driving voltage Vout2/y are alternately applied to the pixel PX through the double-throw switch in the internal driving circuit. The positive driving voltage Vout1/y is the positive voltage Vout1 output by a positive voltage generator (e.g., Amplifier1 in FIG. 1A) divided by a data-dependent variable divisor y using a voltage divider (not shown). The negative driving voltage Vout2/y is the negative voltage Vout2 output by a negative voltage generator (e.g., Amplifier2 in FIG. 1A) divided by a data-dependent variable divisor y using a voltage divider (not shown).
In each pixel on an LCD panel, the amount of light that is transmitted from the backlight through the LCD layer depends on the voltage (Vout1/y or Vout2/y) applied to the pixel. The amount of light transmitted does not depend on whether that applied voltage is negative (Vout2/y) or positive (Vout1/y). However, applying the same polarity of voltage to the same pixel PX for a long period of time would damage the pixel PX. In order to prevent damage, the internal driving circuit of the LCD displays quickly alternate the voltage between positive and negative for each pixel PX, which is called pixel inversion. In order to generate the alternating positive and negative pixel-driving voltages (Vout2/y, Vout1/y)., the internal driving circuit conventionally receives a plurality of negative driving voltages from the DC to DC converter and a plurality of positive driving voltages from the DC to DC converter at all times during operation of the LCD. Switches, (e.g., the double throw switch SW2), within the internal driving circuit alternately connect each pixel PX to the positive driving voltage Vout1/y and the negative driving voltage Vout2/y. Ideally, rapid polarity inversion isn't noticeable because every pixel has the same brightness whether a positive or a negative voltage is applied.
Referring to FIG. 1B a conventional double-throw switch can be implemented within the internal driving circuit as two single-throw (ON/OFF) switches connected in parallel to the same (output) node. Single-throw (ON/OFF) switches can be implemented as mechanical devices, or by semiconductor transistors. In internal driving circuits for LCD panels, ON/OFF switches are typically implemented as field effect transistors (FETs) formed in an integrated circuit. The double-throw switch SW is controlled by a control signal output by a conventional pixel polarity inversion mode controller 1238. The pixel polarity inversion mode controller 1238 controls the double-throw switch SW to alternately select the positive driving voltage Vout1/y and the negative driving voltage Vout2/y according to a predetermined inversion pattern (e.g., Line-paired RGB sub-pixel dot-inversion, line (e.g., row) inversion, frame inversion, etc.). In the first inversion mode, the double-throw switch SW selects the positive driving voltage Vout1/y and the second inversion mode the double-throw switch SW to selects the negative driving voltage Vout2/y.
A mobile display driver IC (Mobile DDI) may additionally include built-in non-volatile memory cells for storing gamma, configuration, and user settings, and thus the DC to DC converter may further be configured to output high voltages associated with erasing and programming non-volatile memory cells. The plurality of voltage outputs of the DC to DC converter may be implemented by one or more amplifiers, charge pumps, or voltage regulators. The internal driver circuit is regarded as a load of the voltage generator(s) of the DC to DC converter.
In designing the DC to DC converter, output voltage ripple and voltage drop are issues that need to be addressed because they can compromise operation characteristics and display quality. To mitigate the output voltage ripple and voltage drop, a voltage-stabilizing circuit comprising a plurality of capacitors is typically provided between the DC to DC converter and the internal driving circuit of the LCD. In a conventional stabilizing circuit, a capacitor is connected to each of the plurality of negative voltages output from the DC to DC converter and additional capacitors are connected to each among the plurality of positive voltages output from the DC to DC converter.
FIG. 1A is a circuit diagram of a conventional stabilizing circuit 12 connected to a positive and a negative output of a DC to DC converter of an LCD display. Referring to FIG. 1A, the conventional stabilizing circuit 12 comprises a second positive-stabilizing capacitor C2P for stabilizing a second positive voltage Vout1 output by Amplifier1, and a second negative-stabilizing capacitor C2N for stabilizing a second negative voltage Vout2 output by Amplifier2. In a typical LCD, the second positive voltage Vout1 and the second negative voltage Vout2 will have the same magnitude, but opposite polarities, relative to ground. And thus, second positive voltage Vout1 stabilizing capacitor C2P can have the same capacitance as the negative voltage Vout1 stabilizing capacitor C2N, and so the stabilizing capacitors C2P and C2N can be implemented by identical capacitors.
Each of the voltage stabilizing capacitors C2N, C2P is typically an exterior capacitor not formed on the integrated circuit(s) that comprise the internal driving circuit and/or the DC to DC converter. The exterior capacitors take up physical space in three dimensions on a printed circuit board outside the LCD panel, and tend to increase the size and weight of products containing LCDs, as well as increasing the part count and the production cost of such products.